How to choose the most suitable biological wastewater treatment

Most suitable biological wastewater treatment process according to the application

There are different discharge restrictions for wastewater produced by different sectors and industries. Treatment systems must be designed around each application to specifically target and treat the pollutants they produce. Therefore, it is important to choose the appropriate biological treatment process for secondary wastewater treatment.

When selecting treatment methods for wastewater treatment plants, there are many options for different stages of treatment.The secondary stages usually consist of biological wastewater treatment systems, and these, in particular, have several different options. There are multiple aspects to consider when choosing the most optimal solution for a particular application.

At the heart of the biological wastewater treatment process, there are three general considerations and two specific to biological treatment.

General considerations include:

  • Occupied area or area occupied by the system on site;

  • Construction costs, how much money will be needed to build the system;

  • Running costs, the costs associated with the daily use of the system

The specific considerations are hydraulic retention time (HRT) and sludge retention time (SRT). THS refers to how long the effluent is exposed to the process. SRT however, is the length of time that a sludge unit (or other bio media) is active in the reactor.

The choice of treatment and the design of biological wastewater treatment systems take into account these five criteria (and others). Often in the decision-making process, weight values ​​are assigned to each criterion and then points are awarded to each method for each of those criteria (often by means of a ranking system). The points are multiplied by the weights and the results are used to compare each option numerically. The weighting system is usually subject to change based on criteria that a business or municipal organization considers more important. Therefore, we will not go into the details of this article.

Instead, we’ll rank the options relative to each other assuming all organic loads and flow rates are the same for every possible system.


We will discuss four biological wastewater treatment processes and their specific advantages and disadvantages below.

Four possible biological processes:

Activated sludge treatment process (ASP)

A suspended solids system that aerates a bulk biological medium and is then pumped to a clarification tank to settle. The sludge from the clarifier is returned to the reactor, called back activated sludge.

Occupied zone: Second highest THS: Second lowest, 4-10 hours

Construction costs: Second highest SRT: Lowest

Operational costs: Low

Batch Sequencing Reactor (SBR)

A version of an ASP that does not have a separate clarification tank. Instead, after a sufficiently long retention time, the solution is allowed to settle and the resulting supernatant is pumped to tertiary treatment.

Occupied zone: Second smallest THS: Second highest, 6-12 hours

Construction costs: Second lowest SRT: Similar to OD

Operational costs: Low

Oxidation ditch (OD)

Another version of ASP that moves effluent around an elliptical or circular channel during an extended aeration cycle.

Area occupied: The biggest HRT: More high 12-24 hours

Construction costs: Highest SRT: Similar to SBR

Operational costs: Low

wastewater treatment

Moving Bed Biofilm Reactor (MBBR)

A filmstrip system that uses a biofilm that has adhered to small, specialized supports that are moved around the reactor by aeration bubbles. Like ASP, MBBR is followed by a clarification stage.

Occupied zone: Smallest THS: Lowest hours 1-5

Construction costs: Lowest SRT: Highest

Operational costs: Low






Solid in suspension

  • Common and conventional

  • Reduced odor

  • Sludge recirculation

  • Bad for shock loads and toxic shock

  • Bad settling


Solid in suspension

  • No separate clarifier tank

  • Remote control

  • Flexibility of operation

  • Sludge recirculation

  • Frequent sludge disposal

  • Increased energy consumption

Beef. Pit

Solid in suspension

  • Low sludge yield

  • Can withstand shock loads

  • Sludge recirculation

  • Can be loud and smelly


Dynamic filmstrip

  • Once through the process

  • Self-regulating

  • Ideal for system upgrades

  • Can handle load and toxicity flows

  • May need a longer start-up period if bioseed is not used

Each of these treatment methods has a variety of advantages and disadvantages that can make them good or bad for a particular application. All four are inexpensive to operate, but construction costs were higher for reactors that required more land, with the oxidation gap being the highest in this regard.

Regarding retention times, the MBBR was better in both cases, with higher sludge retention without sacrificing hydraulic retention like the SBR and oxidation ditch methods. MBBR is a strong contender for consideration, especially if the unit is from a company with a well-designed support stand. In many cases, the comparison of reduction rates an MBBR unit with a lower active surface support than other treatments may indicate that MBBR has lower removal efficiency. Therefore, MBBR can also be considered as a complementary process for use with an activated sludge treatment process to improve the treatment efficiency of the whole system.

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